Converters or vaporizers are widely used in various fields to convert cryogenic liquids to gases. Typical of such liquids are O.sub.2 and N.sub.2 which are normally in the gas state at ambient conditions so that certain vaporizers are able to utilize no more than ambient air to supply sufficient heat to vaporize the liquid. The advantage of dealing with cryogenic liquids is, of course, well known since in the liquid state, the product can be stored, transported, pumped, etc. to the final use site and then vaporized to yield extremely large volumes of gas.
One particular use of such vaporizers, and the one to which this invention is particularly suitable, is for the introduction of an inert gas at high pressure into oil wells for fracing or other treatment. Such uses of high pressure inert gases are shown in Plummer U.S. Pat. No. 3,100,528 and one of the preferred gases for oil well treatment is nitrogen (GN.sub.2).
A typical commercial unit for supplying GN.sub.2 may be provided in the form of a trailer truck which includes the LN.sub.2 storage container, a converter to convert the LN.sub.2 into gaseous GN.sub.2 and associated pumping means to pressurize the gaseous GN.sub.2 for introduction into the oil well. The motive power to operate the pumping equipment may be an internal combustion engine, such as a diesel engine, or may include a gas turbine. In any case, such motive power is also mounted on the trailer truck itself so that the entire unit for providing high pressure GN.sub.2 can be readily transported to the site of the oil well and is mobile to be moved to another location when that particular treatment is completed.
Vaporizers generally may be of the ambient air type, see U.S. Pat. No. 3,672,446 of Tibbets et al, or may have some source of additional heat for vaporization supplied from means such as a gas fired burner, or even by passing engine exhaust directly upon the tube containing the liquid to be vaporized. The need for additional heat depends upon the particular application, i.e. flow of gas desired and the typical ambient conditions.
Ambient air vaporizers, which are suitable for certain types of uses, are generally slow means of vaporization and, of course, are dependent substantially upon the temperature of the ambient air. In addition, the size of the heat transfer area must be extremely large so that the maximum amount of ambient air may contact the heat transfer surface to thus give up heat to the cryogenic liquid flowing through pipes or tubing. Therefore, a large area on location must be available for such vaporizers, thus the ambient air type of vaporizer is unsuitable in locations where space is at a premium, particularly when it is desirable to move the vaporizer from one location to another. Those vaporizers utilizing some heating means include fuel-fired combustion chambers which pass the hot products of combustion over tubing containing the cryogenic liquid to be vaporized. One difficulty with fuel-fired vaporizers is, however, the danger associated with fire or open flame. The danger is especially acute in off-shore oil rigs due to the inaccessibility of the installation.